362 Annals of the Carnegie Museum. 



This society consists in part of species, which are representative of 

 the sand-plain, and this fact in connection with the sandy condition of 

 the soil, which is in places comparatively free from humus, indicates 

 a close similarity between this habitat and portions of the sand- plain. 

 However there are invading groups of Myrica and scattering indi- 

 viduals of Salix cordata, so that a final occupation of the habitat by the 

 shrub-formation is indicated. 



The CotnpositcE, mentioned above as secondary species in the Fra- 

 garia virginiana society, reach their most conspicuous development 

 only in the autumnal aspect, when that society is overshadowed by 

 the Aster ericoides society. 



The Rhus-Alnus Formation. 



The Myrica-Salix thicket, as is evident at this stage, and still more 

 evident in the next stage of the formation, is supplanted eventually by 

 a shrub-formation, which is in reality rather intermediate between 

 thicket and forest. The formation is mainly composed of shrubs and 

 small trees and its vegetational structure is as follows : 



Fades. — 



Alnus incana, Rhus typhina. 



Principal Species. — 



Vitis vulpitia, Cornus stolonifera, 



Solidago canadensis, Cornus amomum. 



Secondary Species. — 



Salix cordata, Salix discolor, 



Salix nigra, Fragaria virginiana, 



Rubiis allegheniensis, Acer saccharinum, 



Dryopteris thelypteris. Toxicodendron picbescens. 



During the earlier stages of this formation, and before the taller 

 growing species have become large enough to shut out the light, the 

 principal and secondary species flourish, but with the maturity of 

 the facies and the consequent development of a more or less dense 

 but low " forest cover," " there is a corresponding disappearance or 

 rearrangement of these smaller species. When typically well developed 

 the formation consists of a dense growth of either the Alnus incana 

 consocies or the Rhus typhina consocies, or a consocies composed of a 

 mixture of the two facies, the whole forming a zone just inside of the 

 zone of cottonwoods. 



" Pinchot, Gifford. " A Primer of Forestry." Part I.— The Forest. U. S. Dept. 

 Agriculture, Div. Forestry, Bull. 24 : 11. 1900. 



